A carbon nanotube (CNT) refers to a nanoscale tubular structure composed of six-member carbon rings whose bonding patterns create a hexagonal lattice that closes upon itself to form the walls of the cylindrical tube structure. Carbon nanotubes are allotropes of carbon that can have a length-to-diameter ratio of up to 28,000,000:1. These cylindrical nanostructures have novel properties that make them potentially useful in nanotechnology, electronics, optics, materials science, and architectural applications. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Numerous techniques have been developed to produce nanotubes including arc discharge, laser ablation, high-pressure carbon monoxide (HiPCO), and chemical vapor deposition (CVD). Most of these processes take place in vacuum or with specific process gases. CVD growth of CNTs can occur in vacuum or at atmospheric pressure and are capable of being synthesized in large quantities using these methods.
The use of conventional reverse osmosis (RO) technology for desalination and filtration is well known. The high initial capital, operating costs, and energy requirements of RO desalination have restricted its large-scale exploitation to specific situations where there are limited sources of fresh water; for example large seafaring vessels and arid desert locations with access to an abundance of low cost energy, such as the Persian Gulf region. Moreover, conventional RO membranes have low specific flow rates due to low water mobility and high pore tortuosity. As a result, conventional RO membranes typically require relatively large, power-intensive systems for desalination. High processing pressure requirements necessitating the use of costly pumping systems and high pressure corrosion resistant piping systems also contribute to the very high capital costs associated with conventional RO desalination installations.
Conventional RO membranes are made of swollen hydrophilic polymers. Under an applied pressure gradient, water molecules move through the membrane by sequential displacement of one another in wet interstices between polymer chains. The interstices are too small for most ions to pass through. The driving force for permeation for membrane separation is the net pressure across the membrane, which is defined as the feed pressure minus the permeate or back pressure, less the difference between the osmotic pressure of the feed and the osmotic pressure of the permeate.
Reverse osmosis membrane flux rates (gallons of filtered fluid produced per an effective area unit of membrane) are typically quite low for commercial RO membranes, resulting in immense surface area requirements for such membranes. The low specific flow rates of such membranes, measured in GFD/psi (gallons of fluid per square foot of membrane area per day, per psi of net driving pressure) also necessitate a large amount of effective membrane area and a high operating pressure to obtain adequate quantities of water. Conversely, membranes using nanofiltration operate at significantly lower pressures than conventional RO membranes and have inherently higher flux rates, typically 5-6 times higher than those for RO membranes (0.11 GFD/psi vs. 0.02 GFD/psi) used during desalination.
High-selectivity of water- vs. ion-transport has not yet been demonstrated for CNT membranes at seawater salt concentrations. Various approaches to filtration, desalination, or colloidal separation using CNTs have been investigated. Accurately and efficiently matching the pore entrance of such CNTs to a target molecule or ion size is important to attain more efficient molecular sieving and/or ionic interaction with selective solutes, molecules or colloidal particles filtered through the CNT. It is difficult to directly manufacture CNTs having precise predetermined pore sizes. Therefore, a need exists for a CNT that can provide a uniform or highly selective pore configuration for use in a number of applications such as chemical separation, desalination, and wastewater remediation. Accordingly, there is a need for a nominally macrocyclic molecular ion-exclusion pore aperture linked CNT that is robust and highly stable in order to facilitate efficient filtration, desalination, and similar particle separation applications.